If you're not backing a sail then ROW is determined by the regular ROW rules - port/starboard, windward/leeward, etc.

Common for RC yachts which don't have the facility to back a sail - but who often encounter weed attaching to keel fin, which seriously affects performance. It is common to sail htw then steer in reverse to wipe the weed off.  The maneuver is often performed just as a precaution before the start sequence. Always approach htw from starboard tack so ROW is maintained while reversing and holding htw. So long as you don't pass htw, you are still on starboard tack with ROW. Not an easy maneuver to initiate without the ability to back a sail so not a successful maneuver at the start.

Backing over the starting line without backing a sail is still subject to 21.1.

A boat sailing towards the pre-start side of the starting line or one of its extensions after her starting signal to start or to comply with rule 30.1 shall keep clear of a boat not doing so until her hull is completely on the pre-start side.
When 21 applies, port-starboard, windward-leeward, and ahead-behind do not apply.

Thank's for the comments. Perhaps my question wasn't precise enough, so here is the scenario: 30 seconds before the gun goes off (starting on flag P) I sail around the commitee boat on the course side, go in front of the pack and then go backwards without touching my sail until I'm behind the starting line. Everybody else is astern and has to keep clear. Am I allowed to do that?

Perfectly legal maneuver. So long as you are behind the line at the start signal, you are probably ROW - but beware of any boat astern. When you reverse into an overlap, you become Keep Clear as windward (R11) and since your action may have given the other boat ROW, you may not be protected by R15 (initially give Room to Keep Clear) - although that might be difficult for the other boat to prove, given that you have reversed and she has moved forward. Anyone have a comment on R15 in this scenario?

1. If you are backing into a wall of boats approaching the starting line, do not forget RRS14, Avoiding Contact.
Even with ROW, you are required to take action to avoid contact, as soon as it becomes clear that the wall cannot keep clear of you.
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2. If you are backing down, with main boom flapping side to side, someone in the crowd approaching you is going to reasonably believe that you backed the sail at some point. If there is doubt in the protest hearing, I give you slim odds of success.

Have you thought sufficiently about how you are going to manage the transition from backing down more or less head to wind to close hauled and at full speed crossing the line on the gun? I would think bearing away onto a close hauled course whilst fulfilling your obligation to keep clear of leeward boats may be rather challenging, especially as the leeward boats will be determined to push you over the line.

On it's face it sounds like a legal maneuver, but unlikely that you'd be able to back fully below the line without establishing overlaps and becoming Keep Clear on boats to leeward.

Difficult to see how this move would be used to advantage, at least in fleet racing. But be sure to get video if you try it...  ;-)